1. Field of the Invention
The present invention relates to a fuel cell vehicle including a fuel cell capable of generating electrical energy by electrochemical reaction of a reactant gas and outputting the electrical energy to a rotating load, a rechargeable energy storage, a switching mechanism for electrically connecting the fuel cell to the rotating load and the energy storage or electrically disconnecting the fuel cell from the rotating load and the energy storage, and a DC-DC converter connected to the energy storage. Further, the present invention relates to a method of controlling the fuel cell vehicle.
2. Description of the Related Art
A fuel cell is a system that supplies a fuel gas (chiefly containing hydrogen) to an anode and supplies an oxygen-containing gas (chiefly containing oxygen) to a cathode to induce electrochemical reaction for generating DC electrical energy.
For example, a solid polymer fuel cell employs a polymer ion exchange membrane as an electrolyte membrane. The electrolyte membrane is interposed between an anode and a cathode to form a membrane electrode assembly. The membrane electrode assembly is sandwiched between separators to form a power generation cell. In use, generally, a predetermined number of power generation cells are stacked together to form a fuel cell stack.
The fuel cell can be mounted in a vehicle such as an automobile, and such a fuel cell vehicle is drawing great attention. Advantageously, the fuel cell vehicle can generate electrical energy efficiently without noxious emissions.
In the fuel cell vehicle, in order to assist the output responsiveness or the like of the fuel cell, a hybrid power supply system which additionally uses a energy storage such as a battery or a capacitor (electric double layer capacitor) is adopted. For example, Japanese Laid-Open Patent Publication No. 2002-118981 discloses a direct power supply including a fuel cell. As shown in FIG. 10, a fuel cell 1 and a battery 2 are connected in parallel to an inverter 3. The electrical energy supplied to the inverter 3 is converted to three-phase alternating current (AC) electrical energy, and supplied to a synchronous motor 4. Thus, wheels 5L, 5R connected to the synchronous motor 4 are rotated.
A DC-DC converter 6 is provided between the battery 2 and the inverter 3. The DC-DC converter 6 is a DC voltage converter, having a function of adjusting the DC voltage inputted from the battery 2, and outputting the adjusted voltage to the inverter 3, and a function of adjusting the DC voltage inputted from the fuel cell 1 or the synchronous motor 4, and outputting the adjusted voltage to the battery 2.
According to the disclosure, the maximum output ratio between the fuel cell 1 and the battery 2 is determined such that the output of the fuel cell 1 falls within the range of 65% to 80% of the overall output. Thus, the loss in the DC-DC converter 6 is minimized, and it is possible to achieve high energy efficiency.
In the conventional technique, during discharging of the battery 2 (during running), the input of the DC-DC converter 6 is set on the side of the battery 2, and the output of the DC-DC converter 6 is set on the side of the inverter 3. The output voltage VPIN matches the output voltage VFC of the fuel cell 1.
During charging of the battery 2 (during regeneration), since the output current IGC of the fuel cell decreases, as shown in FIG. 11, the output voltage VFC of the fuel cell 1 increases, and the input voltage VPIN of the DC-DC converter 6 on the side of the inverter 3 also increases. Thus, in the DC-DC converter 6, in order to maintain the certain voltage difference between the output voltage VBATT on the side of the battery 2 and the input voltage VPIN on the side of the inverter 3, during power regeneration, switching operation is carried out continuously.
Therefore, the switching loss occurs in the DC-DC converter 6, and part of the regeneration energy is lost. Consequently, the fuel economy is degraded.